Mention --select-usb and --select-tcpip in README

PR #3005 <https://github.com/Genymobile/scrcpy/pull/3005>

README.md

@@ -422,7 +422,7 @@ scrcpy -b2M -m800  # short version
 
 #### Multi-devices
 
-If several devices are listed in `adb devices`, you must specify the _serial_:
+If several devices are listed in `adb devices`, you can specify the _serial_:
 
 ```bash
 scrcpy --serial 0123456789abcdef
@@ -436,6 +436,19 @@ scrcpy --serial 192.168.0.1:5555
 scrcpy -s 192.168.0.1:5555  # short version
 ```
 
+If only one device is connected via either USB or TCP/IP, it is possible to
+select it automatically:
+
+```bash
+# Select the only device connected via USB
+scrcpy -d             # like adb -d
+scrcpy --select-usb   # long version
+
+# Select the only device connected via TCP/IP
+scrcpy -e             # like adb -e
+scrcpy --select-tcpip # long version
+```
+
 You can start several instances of _scrcpy_ for several devices.
 
 #### Autostart on device connection